The invention relates to the rebuilding of nuclear fuel assemblies of the kind which comprises a skeleton formed by two end members connected by tie rods attached to the end members and grids distributed between the end members along the tie rods and adapted to maintain a cluster of fuel rods trapped between the end members.
Assemblies of the kind specified are used almost universally in natural water cooled and moderated reactors, more particularly in pressurized water reactors. In some cases an assembly has to be removed and some or all the rods which it contains removed. This situation arises more particularly when the elements of the skeleton of an assembly have become so damaged that an assembly must be rebuilt by removing the rods and replacing them in a new skeleton. Another situation is one in which some or all the rods of an assembly are to be transferred to a new assembly in a different arrangement, or associating the rods with new or different rods (for example, to associate rods containing uranium oxide with rods containing a neutron absorbent, an inert material or a plutonium-enriched fuel). Yet another situation is that in which an assembly modifying the moderating rate is to be constructed.
Different methods of connecting the elements forming the skeleton have been proposed. In the majority of the assemblies at present in operation the tie rods are non-releasably attached to the upper end member and are attached to the lower end member via means enabling the latter to be removed. Some of the assemblies belonging to this group use attachment by a screwthreaded socket which is locked in place by a pin; others use attachment by a screwthreaded socket having a skirt which is subsequently deformed to prevent the socket from rotating. Reference can be made, for example, to French patent application No. 2 521 763 for a description of an assembly of this kind.
In contrast, another group of assemblies, described in French patent application No. 2,536,261, comprises a removable upper end member; this method has the advantage of eliminating the need for tilting the assembly through 180.degree. between its stored position and the position in which the rods are removed.
It is more particularly an object of the invention to provide a method and apparatus which enable a nuclear fuel assembly to be rebuilt by transferring fuel rods taken from an existing, generally heavily irradiated and often damaged assembly to a new skeleton, while using only simple means and limiting the risk of error or breakdown. Another object of the invention is to provide a method and apparatus which is very flexible to use and can more particularly be adapted to the various groups of fuel assemblies mentioned hereinbefore.
To this end the invention provides more particularly a method for rebuilding an assembly of the type hereinbefore defined, wherein the assembly whose fuel rods are to be extracted is disposed in a first vertical chamber, the detachable end member of such assembly being directed upwards; the detachable end member is removed; a second chamber containing and retaining a new assembly skeleton is moved above the end chamber in the same orientation as the assembly of the first chamber and in alignment therewith; the rods to be extracted from the assembly are drawn into the skeleton; and the skeleton is completed by putting the end members in place.
If only the lower end member of the assembly is detachable, the assembly is first placed in the normal position in the first chamber and is blocked in position therein; the chamber is turned over; the new skeleton is moved into an inverted position in its chamber to enable the rods to be extracted from the assembly contained in the first chamber to be drawn upwards towards the skeleton.
As a general rule the two chambers rest on one another via a retaining and centring mask.
It is found that this method of procedure simplifies the handling of the rods and moreover almost completely eliminates the risk of introducing a rod into unaligned passages in the grids of the new skeleton, since the grab which must be lowered through the grids of the new assembly to seize the rods to be drawn upwards ensures guidance.
The new skeleton can have new rods of fissile, fertile, inert or absorbent material which will contribute towards guiding the fuel rods as they rise.
In an advantageous embodiment of the invention the method allows the rebuilding, from a fuel assembly of the kind only the lower end member of which can be removed, of an assembly provided to enable the rods to be extracted after the removal of the upper end member--i.e., an assembly which can be repaired by the use of the apparatus disclosed in French patent application No. 2,536,261.
This procedure begins with a sequence of operations which remains the same, whatever the nature of the assembly of the new skeleton may be. The damaged assembly is deposited in a lower chamber placed in a pool of water and is immobilised in such chamber; the chamber is turned over through 180.degree.; the chamber is lifted in the pool, while remaining submerged, to a level such that a lower plug of the chamber can be removed and a guide mask put in place; the means for attaching the lower end member of the damaged assembly are eliminated, for instance, by cutting off the pin and then removing screws in the case of a usual kind of assembly; the lower chamber containing the assembly without its lower end member is lowered into the bottom position in the pool; an upper chamber, into which a new skeleton without its end members and disposed in the inverted position has already been placed and immobilised, is moved above the lower chamber and aligned therewith; a tool for transferring the rods is positioned above the chamber, and a grab with which it is equipped is lowered through the skeleton to the level of the rod to be extracted and then actuated to seize the rod and lifted into the skeleton, where the rod is released the; operations are repeated until all the rods have been transferred; the lower end member of the new skeleton is fixed; the upper chamber can then be withdrawn and turned over, before its upper end member is put in place.
Once the skeleton of the damaged assembly has been emptied, the operation can be completed by the replacement of the lower end member before removal.
The invention also relates to an apparatus for enabling the process defined hereinbefore to be put into effect, such apparatus advantageously also including means for repairing an assembly in which certain rods are to be replaced, so as to widen the field of use of the apparatus.
The invention will be more clearly understood from the following description of an example of an apparatus forming a particular embodiment of the invention and of the method put into effect in such apparatus. The description refers to the accompanying drawings.